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  Java Practices -> Connection pools
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<div class='page-title'>Connection pools</div>

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<br>Database connections :
<ul>
<li>sometimes take a relatively long time to create.</li>
<li>are always limited in number (most medium- and low-end servers have a maximum
of ~100 connections).</li>
<li>are resources which must be systematically released when no longer in
use. </li>
</ul>

The last point is critical: <span class='highlight'>connections must always be properly released when no 
longer needed.</span> Failure to do so will eventually cause the application to "hang", and fail to respond
to user actions.</li> All code in your application regarding connections should be treated with 
special care.

<P>To ensure that database connections are used as efficiently as possible,
many applications make use of a connection pool, which can sometimes dramatically
improve performance in a distributed application. An application will use
a connection pool as follows :
<ul>
<li>
upon startup, the application creates a pool of database connections (or,
the application's environment may already include a connection pool)</li>

<li>
during normal operation, the application simply fetches a connection from
the pool every time a database operation is&nbsp; performed. This is often
done in a "just-in-time" manner. That is, the application does not keep
a long-lived reference to its connection objects, but will rather fetch,
use, and then close each connection in a single method.</li>

<li>
when the calling code is finished with the connection, it calls a <tt>close</tt>
method, but the underlying connection is not actually closed - rather,
it is simply returned to the connection pool</li>

<li>
when the application shuts down, the connection pool it created upon startup
(if any) is shut down as well</li>
</ul>
A database driver may contain a connection pool as part of its services,
through its implementation of the <tt><a href="http://java.sun.com/javase/6/docs/api/javax/sql/DataSource.html">javax.sql.DataSource</a></tt>
interface. <tt>DataSource</tt> objects are used with JNDI.
<p>The popular <a href="http://jakarta.apache.org/tomcat/">Tomcat</a> tool
implements JNDI, and connection pooling is described in its documentation.
It is not difficult to use.
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction799b-2.html?Id=127'>Get database connection</a> <br>
 
  
  <a href='TopicActionf9c8-2.html?Id=183'>Encapsulate connections</a> <br>
 
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